Two different serotypes of the genus Nitrosomonas were isolated from samples of the sewage plant Heidelberg. These nitrifiers were enumerated in activated sludge of various other sewage plants after immunofluorescent labeling and staining with propidium iodide by flow cytometry. The concentrations of these serotypes of Nitrosomonas spp. were in the range of 0.1 to 2%. Also, a test for the determination of the activity of ammonia-oxidizing bacteria was developed. Nitrite-oxidizing bacteria were specifically inhibited with sodium chlorate, and the activity of ammonia-oxidizing bacteria could be calculated from the increase of nitrite. Concentrations and activities of ammonia oxidizers were measured for a period of 6 months in the sewage plant Heidelberg. With one exception, activities and concentrations of ammonia-oxidizing bacteria decreased and increased in parallel.